Potentially explosive hydrocarbon fuel gases such as propane have been odorized to warn of leaks. Although the use of odorants to warn of leaks of gaseous fuels was first proposed in Germany by Von Quaglios in the 1800s, and odorants were used as early as 1900 in Europe; it was not until 1937 when a school explosion in Texas provided sufficient impetus for promulgation of U.S. laws requiring the addition of an odorant to gaseous fuels. Currently, both natural gas and propane are required to be odorized such that most people can detect the odor at ⅕ the lower flammability limit. For example, 29 CFR 1910.119 (b)(1)(i) states that “liquefied petroleum gases shall be effectively odorized by an approved agent of such character as to indicate positively, by distinct odor, the presence of gas down to concentration in air of not over one-fifth the lower limit of flammability . . . the odorization requirement of paragraph (b)(1)(i) of this section shall be considered to be met by the use of 1.0 pounds of ethyl mercaptan per 10,000 gallons of LP-gas.” The requirements under 49 CFR 173.315 (b) (1) are the same.
Ethyl mercaptan, also known as ethanethiol, is the odorant of choice for 95 percent of the propane industry. However, it must be noted that although tests have shown about 9 out of 10 people can smell ethanethiol at a level of 20 ppb, this still leaves a significant number of people for whom smell is not a reliable indicator of odorant level.
An additional problem known as “odorant fade” was well documented by Beltis in “Characterization of LP Gas Odor and Fade,” Kevin J. Beltis, Consumer Products Safety Commission report CPSC-C-86-1281, June 1986, and it may also reduce the ability to detect leaks. Odorant fade is the loss of odorant effectiveness caused by absorption, adsorption, complexation and/or degradation of the odorant. For example, most odorants can be absorbed by materials with a high surface area, such as soil or dirt. Absorption/adsorption may also occur on the surfaces of new pipes or tanks that have not previously contained odorized propane. Moreover, odorants may be chemically oxidized or otherwise chemically transformed to products (e.g. disulfides) that do not have the same degree of odor warning capability. In particular, rust in tanks is known to cause thiols (mercaptans), such as ethanethiol to oxidize to compounds of lower odor. Collectively, these absorption, adsorption, complexation and degradation phenomena are known as odor fade. Due to odor fade, there remain cases where there is doubt about the amount of odorant present in commercial propane. A recent example is the controversy about odorization levels in LP gas supplied to customers in Massachusetts and Connecticut that was reported in the March 2011 issue LP Gas Magazine.
Leakages of inadequately odorized gas present a high risk of inadvertent ignition and explosion since the ability to detect such leaks is diminished. Thus, there is a need to verify that propane fuel in fact contains the proper level of odorant. The three most common methods of testing for propane odorant are a) the “sniff” test, b) stain tubes, and c) gas chromatography. Optical methods are sometimes used in a laboratory setting. Note that odor fade can occur after delivery. Even if the propane was delivered to the supplier's tank with the proper odorant level or the propane was delivered to the customer's tank with the proper odorant level there is no certainty that the propane supplied to the customer's point of use has the proper odorant level. Testing may be needed along the entire supply chain from production to point of use.
The most basic type of test for odorant is simply a sniff test. However, it is well-known that such a test result may be subjective. There are devices that make the test semi-quantitative by diluting the sample with known quantities of air. Examples include the Heath Odorator, and the Bacharach Odorometer, developed in the 1920s. The Odorometer had drawbacks however: it required ambient air for dilution of the odor and the air had to be passed through multiple filters to remove impurities that otherwise could affect the perceived odor intensity.
Stain tubes, or length-of-stain tubes, have been used for the determination of odorant concentration. For example, Sensidyne and Draeger manufacture hermetically sealed thin glass tubes that contain a detecting reagent that produces a distinct color change when a sample of odorized propane vapor is drawn through the tube. If ethyl mercaptan is present, the detecting reagent produces a colored stain that can be measured with a calibration scale that is printed on the tube. Additionally, there is an ASTM standard for such stain tubes (Standard Test Method for Determination of Ethyl Mercaptan in Natural Gas, ASTM D5305, 2007).
Although length-of-stain tubes have a long history and enjoy ASTM Standard recognition, they have not proved fully satisfactory in the field, as the reading is somewhat subjective and the underlying accuracy is insufficient. According to ASTM Standards D1988 and D5305, the accuracy (reproducibility) of length-of-stain tubes for mercaptan measurement in gaseous fuels is plus or minus 20 to 25 percent or more. A previous Bureau of Mines study came to a similar conclusion. Moreover, visual assessment of color change is inherently subjective; some people are unable to distinguish certain colors.
At the more complex end of the analytical scale, gas chromatography can be very accurate in the laboratory, but is too expensive and awkward (bulky equipment and a compressed carrier gas supply are required) for use in the field. Similarly, Fourier transform infrared spectroscopy (FTIR), nondispersive infrared spectroscopy (NDIR), and laser-based optical absorption techniques can be sensitive, accurate, and free from interferences, but they are also complex and expensive procedures.
At least one vendor (Leister Technologies AG, Galileo-Strasse 10 CH-6056 Kaegiswil/Switzerland—See more at: http://www.leister.com/en/) offers a commercial laser diode spectrometer that could be suitable for mercaptan measurement. But this gas detector costs thousands of dollars exclusive of the power supply, sample pump, and sampling handling components.
The need for an inexpensive and portable detector that can monitor odorant concentrations along the entire supply line greatly complicates the development of an odorant meter. It means that an odorant meter cannot be a complex or expensive device that is used only at a production plant or at a supplier's headquarters. The meter must be portable and practical for field use by delivery and service personnel who are normally at the customer's premises.
These and other problems are addressed by the present invention.